from django.db import utils
from django.db.backends.base.features import BaseDatabaseFeatures
from django.utils.functional import cached_property


class DatabaseFeatures(BaseDatabaseFeatures):
    # SQLite can read from a cursor since SQLite 3.6.5, subject to the caveat
    # that statements within a connection aren't isolated from each other. See
    # https://sqlite.org/isolation.html.
    can_use_chunked_reads = True
    test_db_allows_multiple_connections = False
    supports_unspecified_pk = True
    supports_timezones = False
    max_query_params = 999
    supports_mixed_date_datetime_comparisons = False
    autocommits_when_autocommit_is_off = True
    can_introspect_decimal_field = False
    can_introspect_positive_integer_field = True
    can_introspect_small_integer_field = True
    supports_transactions = True
    atomic_transactions = False
    can_rollback_ddl = True
    supports_atomic_references_rename = False
    supports_paramstyle_pyformat = False
    supports_sequence_reset = False
    can_clone_databases = True
    supports_temporal_subtraction = True
    ignores_table_name_case = True
    supports_cast_with_precision = False
    uses_savepoints = True
    can_release_savepoints = True

    @cached_property
    def supports_stddev(self):
        """
        Confirm support for STDDEV and related stats functions.

        SQLite supports STDDEV as an extension package; so
        connection.ops.check_expression_support() can't unilaterally
        rule out support for STDDEV. Manually check whether the call works.
        """
        with self.connection.cursor() as cursor:
            cursor.execute('CREATE TABLE STDDEV_TEST (X INT)')
            try:
                cursor.execute('SELECT STDDEV(*) FROM STDDEV_TEST')
                has_support = True
            except utils.DatabaseError:
                has_support = False
            cursor.execute('DROP TABLE STDDEV_TEST')
        return has_support
